Hey Mart. This one is almost too easy.

I have had a couple of the Greener police shotguns, one of which I still own, and the conversion is simple.
The perfect length for a chamber is probably 2.5" as the 2.75 needs some help to remove the case after firing. Martini geometry.
The idea for a ball gun sounds great +1

See what you can do with the stock design, I fire standard fosters through mine and the recoil is brutal

Good luck with the project.

Tom